I never really cared for the chemistry involved in synthetic organic colorants as it seemed so repetitive and somewhat confusing from colorant to colorant but that in itself is important in order to understand that nature of this class of cosmetic ingredients. As you will see, many of them are quite similar and the chemistry of each of them is almost the same. So why is there such a big difference in the actual color when the chemistry is so close? I will examine some organic colorants and show the similarities and differences that can exist between some of them.
One simple example is D&C Orange 4 and FD&C Yellow 6. D&C Orange 4 is a colorant that is only allowed for external applications mainly for facial cosmetics and mostly for cosmetic powders. FD&C Yellow 6 is a food color that can be used in almost any cosmetic application except eye area use. Their hues are almost identical to one another and this fact aided me some years ago when I had a color availability issue with D&C Orange.